1. Field of the Invention
The invention relates generally to integrated circuit semiconductor devices and more specifically to highly integrated implementations of global positioning system receivers.
2. Description of the Prior Art
The retail price of complete global positioning system (GPS) receivers, including hand-held, battery-operated portable systems, continues to decrease. Competitive pressures drive manufacturers to reduce construction costs while maintaining or actually improving functionality and reliability. Semiconductor fabrication advances have provided a vehicle to meet such goals, and to offer still smaller devices.
Semiconductor fabrication processes are not always uniform from batch to batch. Small process inconsistencies can manifest as variances in transistor characteristics, e.g., collector to base capacitance, also known as the Miller capacitance. In an integrated circuit implementation of a voltage controlled oscillator (VCO), such process variances can result in VCOs that fail to start oscillations due to insufficient loop gain. Capacitors can be added to increase positive feedback, but such capacitance additions can be excessive in the opposite extreme of process variation. A positive feedback capacitance that varies directly with the process variation is needed for high yield manufacturing of reliable components.
Dual-frequency carrier GPS receivers typically track a pair of radio carriers, L1 and L2, associated with GPS satellites to generate accumulated delta-range measurements (ADR) from P-code modulation on those carriers and at the same time track L1 C/A-code to generate code phase measurements. Carrier L1 is positioned at 1575.42 MHz and carrier L2 is positioned at 1227.78 MHz. Each carrier is modulated with codes that leave the GPS satellite at the same clock time. Since the ionosphere produces different delays for radio carriers passing through it having different radio frequencies, dual carrier receivers can be used to obtain real-time measurements of ionospheric delays at a user's particular position. The L1 and L2 ADR measurements are combined to generate a new L1 ADR measurement that has an ionospheric delay of the same sign as the ionospheric delay in the L1 pseudorange. Accurate ionospheric delay figures, if used in a position solution, can help produce much better position solutions. Without such real-time ionospheric delay measurements, mathematical models or measurements taken by third parties (which can be old) must be used instead.
With a highly-integrated GPS receiver implementation, it is desirable to incorporate a dual-conversion frequency plan that allows one set of intermediate frequencies (IF) to be processed, regardless of whether L1 or L2 has been selected. In a highly-integrated GPS receiver implementation, it is desirable to share one PLL that can generate both the required frequencies for the two carrier frequencies used in the dual conversion process.
The relative frequencies of the L1 and L2 carriers allow a super-heterodyne type of receiver to generate a first local oscillator (LO) frequency that is approximately midway between L1 and L2 such that it will produce near identical intermediate frequencies (IF). For L1, the difference between L1 at 1575.42 MHz and LO at 1401.6 MHz produces a first IF of 173.82 MHz. For L2, the difference between L2 at 1227.6 MHz and LO at 1401.6 MHz produces a first IF of -174 MHz. Therefore, to select L1 or L2, a radio frequency (RF) filter for each is switched in or out in front of the RF amplifier or in front of the first mixer. A second LO frequency of 186.88 MHz is desirable because such a frequency applied to a second mixer produces a second IF of 13.06 MHz for reception of L1, and 12.88 MHz for reception of L2. Preferably, both the first and second LO frequencies are produced by a single VCO, in order to save cost and in order to preserve phase coherence through two stages of conversion.